The present invention relates to a composition which can be foamed or expanded, using as the sole blowing agent a filler precursor of hydrated alumina which loses its water of hydration upon heating, and to articles formed from said composition.
The present invention further relates to an improved method of forming such articles from foamed or expanded polyolefin materials by direct extrusion.
Generally, thermoplastic polymeric materials may be heated to a plastic or molten condition, foamed and expanded, and then extruded into articles of desired shape. The unexpanded polymeric material may be provided, for example, in the form of beads, pellets, or other preformed particles which are coated or otherwise combined with a suitable chemical blowing agent. When such preformed particles are heated, the blowing agent generates a gas as it decomposes and thereby causes the polymeric material to foam and expand as it is converted into a molten condition. This expanded molten material may be then extruded and quenched to provide articles having a desired cellular structure.
With most expandable polymeric materials, the density of the foam, and the resulting extruded article, depends upon the amount of blowing agent which has been employed. Expandable polyolefin materials are an exception, however, since conventional extrusion procedures do not facilitate the formation of articles having a specific gravity of less than about 0.45, regardless of the amount of blowing agent used.
While a complete explanation is lacking, it appears that perhaps molten polyolefin materials cannot contain the expanding gas as rapidly as it is generated. Thus, when attempting to form polyolefin articles of such low specific gravity by direct extrusion, the expanding gas ruptures the surface of the molten material as it is extruded and causes collapse of some of the cellular structure. The polyolefin article which results usually exhibits a rough and porous surface and is of greater specific gravity than desired.
U.S. Pat. No. 4,029,612 discloses the use of silica-promoted chemical blowing agents and aluminum hydroxide (up to 20 weight percent) in thermoplastic polymeric materials comprising polyolefins which can be extruded or injected to form foamed articles. U.S. Pat. No. 3,563,939 discloses the use of particular types of alumina trihydrates as fillers for organic compositions comprising polypropylene and ethylene-propylene copolymer, without a blowing agent, which are used as electrical insulating materials. Although the preferred organic binder system of U.S. Pat. No. 3,563,939 for use with a low-alkaline alumina hydrate filler, comprising a polyepoxide and curing agent, is disclosed in the examples and claimed as such, this reference alleges that such a filler could be used with a wide variety of organic binder materials, comprising thermoplastics which include polyethylene, propylene and ethylene-propylene copolymers.